The present invention relates to hermetically sealed electrical feedthroughs and more particularly feedthroughs for use in an enclosure constructed of a high expansion metal such as a minimum.
Hermetically sealed electrical feedthroughs are typically constructed by bonding a connector pin into a tubular ferrule or bushing using a glass-like insulating material, the ferrule being constructed of a metal such as KOVAR having a coefficient of expansion which approximates that of the glassy insulating material. The ferrule is then soldered into a bore in the enclosure bulkhead, each of the parts being previously plated to facilitate soldering. Typically, minimal clearance is provided between the ferrule and the bore in the enclosure wall in order to maintain concentricity which is important in many electrical applications, particularly those involving radio frequency (r.f.) energy. While such constructions have heretofore been reliably implemented where the enclosure is steel or other low expansion metal, extremely high rates of failure have been encountered when attempting to utilize the same constructional techniques in making feedthroughs through an aluminum wall member, owing to the very high coefficient expansion of aluminum.
The present invention is predicated in substantial part upon a perception that most seal failures in aluminum enclosures are due, not to failures of the glass insulating material but, rather, to failures of the solder joint between the ferrule and the material of the bulkhead within which the ferrule is soldered. Soldering a minimal clearance joint between the ferrule and the bulkhead results in a very thin layer of solder. While a thin layer of solder exhibits great strength in certain contexts and types of testing, it does not provide for any significant radial compliance in the context of the ferrule/bulkhead joint described above. Accordingly, when the feedthrough is subjected to temperature cycling, as is required in the testing of many military components, the solder joint is fatigued and fails. This tendency to failure is apparently exacerbated by the well known tendency of solder to creep under stress. Not only is the clearance in general too small to provide adequate radial compliance, but the thickness of solder layer is typically non-uniform since no means is provided for centering the ferrule within the very small clearance of the bore. Thus, the ferrule will typically be somewhat eccentric, producing a solder thickness even thinner than the nominal radial clearance.